1. Field of the Invention
The present invention relates to ophthalmic lenses, and more particularly to the design of contact lenses leveraging and tailoring the resulting strain energy of the eye-lens system when the lens is worn on eye to achieve improved centration, translation, rotation/stabilization, comfort and ultimately vision.
2. Discussion of the Related Art
Contact lenses are considered medical devices and may be worn to correct vision and/or for cosmetic or other therapeutic reasons. Contact lenses have been utilized commercially to improve vision since the 1950s. Early contact lenses were made or fabricated from hard materials, and were relatively expensive and fragile. Although these contact lenses are still utilized, they are not suitable for all patients due to their poor initial comfort. Later developments in the field gave rise to soft contact lenses, based upon hydrogels, which are extremely popular and widely utilized today. The introduction of soft contact lenses has significantly improved the comfort experienced by the wearer. While this achievement is the result of numerous developments and advancements by many in this field, a significant consideration is that soft contact lenses are significantly less rigid than their predecessors. As such, when the contact lens is placed on eye, it is more susceptible to the stresses and strains placed upon it as it deforms and conforms to the anterior surface of the eye. This interaction of the lens with the shape of the eye, in itself is a major consideration, particularly when one attempts to design a lens for a specific purpose such as vision correction, but other considerations for additional purposes are just as important and relevant.
Anatomical shapes of the eye although generally similar, do differ from patient to patient, and also tend to be asymmetric. Specifically the asymmetry of the normal eye can be described as having a larger change in curvature in the nasal region as compared to the change of curvature in the temporal region. Said another way, in looking at the curvature of the peripheral region of the eye in the transverse plane as one moves from the center of the cornea in towards the nose, the rate of change of curvature is larger as compared to moving from the center of the cornea out towards the temple, thus resulting in two different curvatures and resulting in a transverse plane asymmetry. This asymmetry also results in asymmetric forces being placed upon the contact lens when it is worn. Specifically, as the posterior surface of the contact lens interacts with the anterior surface of the eye, particularly in the peripheral region, it can result in the contact lens being displaced temporally thus impacting centration. If the lens is no longer centered, vision correction may be impacted. In the sagittal plane of the eye, it is known that the peripheral inferior portion of a patient's eye tends to be steeper than the peripheral superior portion. Attempts to leverage this sagittal asymmetry are presented in U.S. Pat. No. 6,406,145 in which the innovators take into account the natural shape of the lens wearer's eye in designing the base curve of the lens. While this is a step in the proper direction, one may realize additional improvements by considering the multi-dimensional aspects of the eye's asymmetries. This is accomplished by analyzing and assessing the contact lens and the eye as a system, particularly as it relates to the resulting strain energy and lens centration as a result of the lens geometry interacting with the eye geometry when the lens is worn. Other contributing factors include the mechanical properties of these items as well.
In treating presbyopic patients, one innovation is the use of translating lens designs. As a person ages, the crystalline lens gradually becomes more rigid, and thus their eyes are less able to accommodate. Said another way, their ability to alter the shape of the natural lens to focus on objects is diminished. This condition is known as presbyopia. The typical translating lens relies on the relative movement of the contact lens relative to the eye, specifically the pupil. Typically the translating lens will have multiple optical zones, for instance a near and far zone to account for the loss of the patients ability to accommodate, and depending on the angle of gaze, one can optimize vision by directing the gaze through one zone or the other. To accomplish this, the near and far zones are generally placed inferior and superior respectively, and as an example as one looks downward (typically for near vision needs such as reading) they are looking through the lower (near) portion of the lens. This is successful because the lens, through interaction with the lower eyelid, is typically driven upward, relative to the pupil, whose angle of gaze is being directed downward. As their gaze returns to a more horizontal position and they look to objects in the distance, the relative position of the lens is such that the pupil of the eye is now aligned with the superior (far) portion of the lens. Thus optimizing focus for both near and far vision needs.
The relative movement of the translating lens may be impacted by the asymmetry of the eye and how it interacts with the contact lens thus not allowing the desired result to be achieved. But there are other considerations as well, for example, U.S. Pat. No. 7,216,978 illustrates that the upper and lower eyelids do not move strictly in a vertical direction during blinking. Rather, the upper lid moves substantially vertically, with a small nasal component during blinking, and the lower lid moves substantially horizontally, moving nasal-ward during blinking. Additionally, the upper and lower eyelids are not symmetrical with respect to a plane cutting though the vertical meridian. In other words, individuals do not blink symmetrically relative to a horizontal axis drawn between the open upper and lower lid. Accordingly, blinking in of itself may not result in the ideal translation of the contact lens thus presenting yet another opportunity to improve upon the design. Another type of translating lens has a truncated shape. That is, unlike most lenses that are substantially continuously circular or oval, the lower portion of the truncated contact lens is flattened by cutting off or shortening that part of the lens. This results in a substantially flat, thick edge at the bottom of the lens. Exemplary descriptions of such lenses are set forth in a number of patents, including U.S. Pat. Nos. 7,543,935, 7,430,930, 7,052,132, and 4,549,794. However, a relatively flat edge on contact lenses such as these may tend to reduce comfort. An alternative approach which leverages the concept of a minimum energy position is that which is provided in U.S. Pat. No. 7,810,925, in which a lens design with two discreet stability positions is suggested to optimize lens position for near and distance vision needs. The concept of minimum potential energy position can be leveraged to achieve these two stability positions. Given that some level of displacing force (potentially a significant level in the '925 patent case) is required to move from one position to the other it's likely that some level of discomfort is also being introduced for the initial stability position to be overcome in order to move to the second stability position. Furthermore, the approach of the '925 patent is limited to two distinct positions, in contrast to applicant's invention which not only leverages a continuum of positions and thus relative strain energies rather than distinct states, but also allows for a smooth transition along this continuum.
In the astigmatic patient, in addition to centration, relative rotational orientation of the lens is important to correct one's vision. Astigmatism is caused by a non-rotationally symmetric curvature of the cornea and/or the crystalline lens in the optical zone. A normal cornea is substantially rotationally symmetric, whereas in an individual with astigmatism this is not the case. In other words, the optical zone of the eye is actually more curved or steeper in one direction than another, thereby causing an image to be stretched out into a line of focus (cylinder) rather than focused to a single point. Toric rather than spherical/single vision lenses can be used to address this. A toric lens is an optical element having two different powers in two orientations that are perpendicular to one another. Essentially, a toric lens has one power, spherical for correcting myopia or hyperopia, and one power, cylinder for correcting astigmatism built into a single lens. These powers are created with curvatures oriented at different angles which are preferably maintained relative to the eye. The proper rotational orientation of the toric lens is essential to properly correct for astigmatism. However, toric contact lenses may tend to rotate on the eye thereby temporarily providing sub-optimal vision correction. Accordingly, currently utilized toric contact lenses also include a mechanism to keep the contact lens relatively stable on the eye when the wearer blinks or looks around in order to maintain the correct vision correction. To ensure the proper orientation of the lens, various methods of lens stabilization have been utilized such as ballast or preferential thick and thin zones. While there are various ways to achieve stabilization, all ways ultimately will be affected to varying degrees, by the interaction of the posterior surface of the contact lens with that of the anterior surface of the eye, particularly in the peripheral regions, which may also negatively impact vision and or subjective comfort. The challenge with currently designed or utilized stabilization zones is a tradeoff between contact lens stability and comfort, plus the physical limitations associated with increased thickness. Changes to the design to improve rotational speed, such as increasing the surface slope of the stabilization zone, also increases contact lens thickness and may adversely impact comfort. Additionally, the contact lens design has to accomplish two things; namely, to rotate to the proper orientation on insertion, and to maintain that orientation through the wear period. Conventional designs require tradeoffs in performance between these two modes.
In more recent attempts, for example, see U.S. Pat. No. 8,827,448, use of astigmatism-free customized lenses are proposed for refractive correction with a first cylindrical power on the anterior surface and a second cylindrical power on the posterior surface of the contact lens. While it is suggested that improved visual acuity is achieved with such a design, these items are limited to the optical zone of the lens and how that interacts with an asymmetrically shaped cornea. Design changes in other regions, most notably the peripheral region of the lens, can still have an impact and would not adversely impact those limited to the optical zone seeking to improve visual acuity and thus coexist and further improve lens performance.
Some innovators in this space have attempted to address the lens/cornea mismatch by customized patient specific designs. Specifically, they measure the corneal surface and attempt to match the posterior surface of the contact lens with the topography of the cornea. See for example U.S. Pat. Nos. 6,786,603, 6,340,229 and 6,305,802. This alternative approach while resulting in a near-perfectly matched conformal fit, may not adequately address the issue at hand as it may introduce undesired consequences related to a lack of movement of the lens due to the precise conformal matching of the lens to the cornea. It also can result in a large number of Stock Keeping Units (SKU's) for a system which is costly to both the manufacturer as well as the consumer. In order to maintain a healthy ocular physiology, movement of the contact lens is essential to allow for adequate tear exchange. Humans on average blink about twelve (12) times each minute while awake, each blink can impart movement to a lens, thus facilitating the essential tear exchange to maintain a healthy tear film. But while movement is important for health purposes, too much movement of the contact lens may negatively impact both subjective comfort and vision.
Soft contact lenses for eye enhancement, be they for customized patient specific devices, presbyopia, astigmatism, cosmetic or therapeutic purposes and/or other optical defects or correction, may be further improved by incorporating non-optical features which leverage the principals of optimized strain energy of the system along a continuum as described herein to achieve optimal positioning, movement, orientation and/or stability of the lens while on eye, all of which can positively impact both comfort and vision and perform these functions in a cost effective manner. The prior art devices described above employ features and designs resulting in certain tradeoffs, for example, comfort and fit for visual acuity, lens centration and movement for health and vision, or lens systems with extensive SKU's to adequately address patient variations. Accordingly, there exists a need for contact lenses with improved on eye performance while maintaining eye health as well as a high degree of comfort and vision.